Nice big room, symmetrical, not a square, open behind the listening chair -- I'd say you're going to need minimal treatment, if any, beyond the usual furnishings -- rug, furniture, bookshelves. Perhaps some diffusion at the first reflection point behind the speakers to tame the curved diaphragm focus effect (dipoles don't interact much with the side walls). In my experience, windows are problematic behind dipoles, their contribution will depend on their location. You may find that you want to put in heavy drapes or shutters to cover them when you're listening. As to position, at least three feet from the front wall, two feet from the sides, and on an equilateral triangle with the listener and toed in to face the listening chair should be a good starting point. Then mark the floor and experiment, dipoles are very sensitive to position and the optimal position will depend on the specific room. You'll likely find that the further out from the front wall the speakers are, the more depth they exhibit, and that you have to compromise between best imaging and best bass. (There are formulas such as Cardas for optimal bass response, but in my experience anyway they aren't all that useful with dipoles.)
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I use the clsIIz and my room is roughly 26 by 44 or something like that. It opens into a few open stairwells and another thousand feet of floor space or so. I have a TV instead of a fireplace, but this photo shows what I have done to make my speakers sound great.
If you think this has some bearing on what you are getting at, I will measure it off for you.
In that large of a room, you are probably going to need a larger amp. I use a Sanders ESL amplifier on my CLSIIA's and love it. I tried a Pass XA30.5, it sounded great, but just not enough juice. The XA100.5 monos were fantastic, but way too much money. The Sanders amp is a wonderful product at an excellent price. He has a 30 day trial period. You should really hear one with your CLSes.
Your CLSIIs will play with your 60 watt amp, but soon you will hear the short comings of not enough power. You may want to consider tube amplification. Any tube amplifier with 100 watts or more will do.
I assume you will be placing your speakers on either side of the fireplace. Pull the speakers 3' to 4' out with very little toe in. You will want to hang some window treatment on the windows behind the speakers. Don't worry about room treatment for now. See how the speakers react to your room.
If the speakers will not play loud enough there is a chance the panels are clogged and need replacement. When the panels get dirty from air born contaminants and humidity the efficiency of the speaker drops dramatically. Using the brush attachment with your vacuum cleaner and slowly moving it across the panels on a regular basis is a good practice. Replacement panels are expensive.
You will most likely want to try a subwoofer.
I've thinking of moving up in power at some point. The Audio Matiere is a tube amp and far out does its 60 watts however my plan to to go with a couple of 200 watt monoblocs from Tube Research. I'm not really having any trouble with volume levels but the bottom end could have a little more weight which I'm hope the extra power would help. The panels in these were replaced in 2001 and Martin logan told me the newer replacements are better, lighter and more responsive I don't know if that would be a significant step or not.
When I had CLS's I tried subwoofers with mixed results. I always thought the Vandersteen subwoofer would be a good match. It has a unique way of connecting to the system and relieves your main amplifier of dealing with the lowest octaves. It uses three 8' woofers in a slot loaded configuration. It is very quick yet moves air equivalent to a 15" woofer.
With the cls1 which were more demanding than than yours,I used a spectral amp 60 watts/vintage 1985 with a arc sp9 preamp. I had a large Mac and they nearly blew the speakers apart, that I previouslyhad used with OHM f's . If they are working leave them alone. Although I admit , mine rattled when i played organ music and maybe a subwoofer will add duration. I assume they are 25 years old by now.
I use a Bryston 14BSST/Bechmark DAC-1 Pre with my CLSZ2's and couldn't be more pleased but without a sub I would not own these speakers. IMHO placement requires keeping any solid object including walls at least 3 feet away from the side rails of this speaker and put the sub between and behind the CLS and adjust phase prn.